University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Masters Theses Graduate School 8-2014 Digital-to-Analog Converter Interface for Computer Assisted Biologically Inspired Systems Nicholas Conley Poore University of Tennessee - Knoxville, [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_gradthes Part of the Electrical and Electronics Commons, and the VLSI and Circuits, Embedded and Hardware Systems Commons Recommended Citation Poore, Nicholas Conley, "Digital-to-Analog Converter Interface for Computer Assisted Biologically Inspired Systems. " Master's Thesis, University of Tennessee, 2014. https://trace.tennessee.edu/utk_gradthes/2841 This Thesis is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Masters Theses by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a thesis written by Nicholas Conley Poore entitled "Digital-to-Analog Converter Interface for Computer Assisted Biologically Inspired Systems." I have examined the final electronic copy of this thesis for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Master of Science, with a major in Electrical Engineering. Jeremiah Holleman, Major Professor We have read this thesis and recommend its acceptance: Benjamin Blalock, John D. Birdwell Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) University of Tennessee, Knoxville Trace: Tennessee Research and Creative Exchange Masters Theses Graduate School 8-2014 Digital-to-Analog Converter Interface for Computer Assisted Biologically Inspired Systems Nicholas Conley Poore University of Tennessee - Knoxville, [email protected] This Thesis is brought to you for free and open access by the Graduate School at Trace: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Masters Theses by an authorized administrator of Trace: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a thesis written by Nicholas Conley Poore entitled "Digital-to-Analog Converter Interface for Computer Assisted Biologically Inspired Systems." I have examined the final electronic copy of this thesis for form and content and recommend that it be accepted in partial fulfillment of the requirements for the degree of Master of Science, with a major in Electrical Engineering. Jeremiah Holleman, Major Professor We have read this thesis and recommend its acceptance: Benjamin Blalock, John D. Birdwell Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official student records.) Digital-to-Analog Converter Interface for Computer Assisted Biologically Inspired Systems A Thesis Presented for the Master of Science Degree The University of Tennessee, Knoxville Nicholas Conley Poore August 2014 c by Nicholas Conley Poore, 2014 All Rights Reserved. ii In dedication to my father and mother for making me who I am. iii Acknowledgements First and foremost, I would like to thank my adviser, Professor Jeremiah Holleman, for without his signature this document would have never seen the light. On a more serious note, Dr. Holleman has been a great mentor for the past year. His technical advice was of great value to me, as well as his kind encouragement which helped through many difficult times. Professor John D. Birdwell, my co-adviser, has offered his deep expertise in the area of control systems, and has also given me great support during my time at the Univeristy of Tennessee. For that, I owe him great thanks. I am deeply thankful to Professor Ben Blalock for his great lectures on analog circuit design, for his amazingly broad knowledge that he was always willing to share, and for his enthusiasm and optimism. I again would like to thank Professor Holleman and the ISiS group, as their analog signal processing project gave me inspiration to work on low-power DACs and system interfaces. I would like to thank Mosis for fabricating the chip. Many thanks to DARPA for funding my research assistantship that allowed me to come to graduate school in the first place. I would also like to thank our wonderful administrative assistants, Dana Bryson, Julia Elkins, and Melanie Kelley for all their help. I want to take this opportunity to let my friends and co-workers know how much I appreciate their company and technical advice. Thank you Shahriar\Pollob" Jahan, Kelly Griffin, Tan Yang, Junjie Lu, Terrance Randall, Mohsen Joudi, Peixing Liu, Logan Taylor, and Jake Shelton. iv Daniel Tubb, Morgan Helton, Matt Zarb, Ilona Molotoka, Doscher Shewmake, Ali Ghezawi, Chris Zotti, Andy Dagen, Emert Ridener, and many other friends { you are important to me. To my family, you are the most important thing in my life. Without you my work would be meaningless. Heather Bradley, I cannot thank you enough for your patience and everything you have done for me. The words of gratitude in my thesis may not say much and may not be as eloquent as I would like them to be, but I really hope that even without these words, all of you feel very much appreciated. You truly are. v \Learn from yesterday, live for today, hope for tomorrow. The important thing is not to stop questioning." - Albert Einstein vi Abstract In today's integrated circuit technology, system interfaces play an important role of enabling fast, reliable data communications. A key feature of this work is the exploration and development of ultra-low power data converters. Data converters are present in some form in almost all mixed-signal systems; in particular, digital-to-analog converters present the opportunity for digitally controlled analog signal sources. Such signal sources are used in a variety of applications such as neuromorphic systems and analog signal processing. Multi-dimensional systems, such as biologically inspired neuromorphic systems, require vectors of analog signals. To use a microprocessor to control these analog systems, we must ultimately convert the digital control signal to an analog control signal and deliver it to the system. Integrating such capabilities of a converter on chip can yield significant power and chip area constraints. Special attention is paid to the power efficiency of the data converter, the data converter design discussed in this thesis yields the lowest power consumption to date. The need for a converter with these properties leads us to the concept of a scalable array of power-efficient digital-to-analog converters; the channels of which are time-domain multiplexed so that chip-area is minimized while preserving performance. To take further advantage of microprocessor capabilities, an analog-to- digital design is proposed to return the analog system's outputs to the microprocessor in a digital form. A current-steering digital-to-analog converter was chosen as a candidate for the conversion process because of its natural speed and voltage-to-current translation vii properties. This choice is nevertheless unusual, because current-steering digital- to-analog converters have a reputation for high performance with high power consumption. A time domain multiplexing scheme is presented such that a digital data set of any size is synchronously multiplexed through a finite array of converters, minimizing the total area and power consumption. I demonstrate the suitability of current-steering digital-to-analog converters for ultra low-power operation with a proof-of-concept design in a widely available 130 nm CMOS technology. In statistical simulation, the proposed digital-to-analog converter was capable of 8-bit, 100 kSps operation while consuming 231 nW of power from a 1 V supply. viii Table of Contents 1 Introduction1 2 Background4 2.1 Integrated Circuits............................4 2.1.1 Digital Circuits..........................5 2.1.2 Analog Circuits..........................6 2.1.3 System-on-Chip and Mixed-Signal Circuits...........7 2.1.4 Transistor Operation.......................7 2.2 Analog and Digital Signals........................8 2.3 System Inputs and Outputs.......................9 2.4 DAC Performance Metrics........................ 10 2.4.1 Performance............................ 11 2.4.2 Static Figures of Merit...................... 12 2.4.3 Dynamic Figures of Merit.................... 15 2.5 DAC Methods and Architecture..................... 17 2.5.1 DAC Topologies.......................... 17 2.5.2 Binary Weighted vs. Thermometer Encoding.......... 20 3 Analog System Interface Design 22 3.1 Proposed Front-End Architecture.................... 22 3.2 Proposed Back-end Architecture..................... 32 3.3 Analog Interface Operation Summary.................. 38 ix 3.3.1 Operation Example........................ 38 3.3.2 Time Delay Definitions...................... 41 3.3.3 Error Budget Analysis...................... 43 4 DAC Design and Simulation 45 4.1 Initial design of DAC........................... 45 4.1.1 Binary Weighted Current Steering DAC............ 45 4.1.2 Current Sink Network and Standard Cell............ 46 4.1.3 Switch Design........................... 51 4.2 Binary Weighted DAC Simulation Results............... 56 4.2.1 Static